1. Field of the Invention
The present invention relates to a stator end-winding system of the stator of an electric machine having a array of bars expanding coaxially to the axis of the machine at its end areas in the shape of a hollow cone, and having a means for supporting same including an inside-ring which extends cone-like along a substantial axial portion of the cone-shaped array of bars. It relates also to a retrofitting set for a stator end-winding support system of an electric machine having a array of bars expanding outwards coaxially to the axis of the machine at its end areas in the shape of a hollow cone. It relates further to a method of retrofitting a stator end-winding system of an electric machine having an array of bars expanding outwards coaxially to the axis of the machine at its end areas in the shape of a hollow cone, and a support system for same which includes supporting members located at the inside and outside relative to the cone-shaped array of bars.
The high dynamic forces acting during the operation at locations of resonance of the mechanical stator end-winding system of stator end-windings and the problems resulting therefrom, specifically in the case of large electrical machines, specifically of turbo-generators with a directly hydrogen-cooled stator winding, are generally known. The appearance of exciting forces of specific frequencies can not be influenced at a given operation wherewith it is endeavoured to move the locations of resonance of the mechanical stator impedance as much as possible away from the exciting frequencies occuring during the operation. In operation, operational vibrations, specifically in the case of resonance, lead to high dynamic stresses of the end-windings, specifically in case of turbo-generators, which may lead to a loosening of the entire stator winding assembly. Abrasions of insulation and damages at the stator and the results thereof lead in turn to long shut-down times for a repair of the stators. Due to the known direct hydrogen cooling of the stator winding of generators the number of slots of the stators is kept as low as possible resulting in large dimensions of the bars of the stator and accordingly in large masses. Because the dynamic forces are proportional to the masses acted upon, aside from the acceleration of the vibrations, accordingly high stresses are the result, specifically of the end-winding assembly. Accordingly, there exists on the one hand the problem in the case of the known stator end-winding systems to be able to absorb the large dynamic forces. On the other hand such stator end-winding systems, also if they have been improved regarding dynamic stresses, must be subjected from time to time to maintenance work whereby the generally complicated and, regarding the maintenance, difficult structural designs of stator end-winding systems lead to corresponding shut-down times: A repair of most of the known stator end-windings means a new winding of the stator.
2. Description of the Prior Art
The U.S. Pat. No. 4,488,179 discloses a stator end-winding system, specifically of a turbo-generator, including an array of bars expanding outwards coaxially to the axis of the machine at its end areas in the shape of a hollow cone and a supporting system for same. The supporting system includes a plurality of rings located inside of the hollow cone formed by the array of bars, and at the outside a massive conically extending outside ring. The bars of the winding are braced therebetween by means of tie rods and intermediate rings. The outer massive ring is, furthermore, axially anchored at the stator body. Due to the generally used cone-shaped array of the bars which expands axially towards the outside it is not possible to revise or repair, respectively the stator end-winding support system without a dismounting of the bars which means a new winding of the stator. Furthermore, a plurality of weakly dimensioned inside rings lead to a merely low stiffness or rigidity, respectively of the stator end-winding unit because the bracing forces act more or less only along straight lines and do not act uniformly at the cone of the stator bars.
The U.S. Pat. No. 4,525,642 discloses a stator end-winding again with an array of bars expanding conically outwards and an anchoring between the body of the stator and the outer support of the bars at the stator end-winding in order to absorb thermally caused expansions between the arrangement of bars and the body of the stator.
U.S. Pat. No. 4,563,607 discloses a stator end-winding again with an array of bars expanding outward coaxially to the axis of the machine at its end areas in the shape of a hollow cone and a support for same as well, which latter, located at the inside includes clamps extending under the bars and holding same and being radially braced to upper plates which are anchored at the body of the stator and are substantially axially aligned. The plates and the clamps are braced together by means of bracing rings located between the bars of the bar array. The support is mounted to the stator body. The small masses of the inner and outer members used here for the support and braced against each other have a negative effect upon the damping of the resonance behaviour of the stator end-winding such that, together with the line-contact bracing only a unsufficient rigidity of the stator end-winding is achieved.
U.S. Pat. No. 4,808,873 discloses basically to insert a suitable padding material between the bars of the array of bars.
The European Patent document EP-A-0 309 096 discloses further to forsee an outer support in the form of an outer ring at a stator end-winding having again a array of bars expanding in the form of a hollow cone in order to embrace the bars by tensioning bands extending from said outer ring. Again, due to the small installed supporting masses and line-contact like bracings the resonance behaviour of this stator end-winding is unsatisfactory due to the lack of rigidity. Additionally, due to the fact that the ring is displaced axially inwards from the largest outer diameter of the cone of the arrangement of bars a revision of the stator end-winding is not possible but rather only a disassembly along with at least a partial removal of the bars.
Now, the Swiss Patent Specification 634 695 discloses a stator end-winding of an electric machine having an array of bars expanding outwards coaxially to the axis of the machine at its end areas in the shape of a hollow cone, whereby a support is provided which includes an inside ring which extends cone-like along a substantial axial portion of the cone-shaped array of bars, and includes an outside ring system at the end of the arrangement of bars which is radially braced onto the inside ring by means of tie rods via the array of bars. In order to make the maintenance activity of the stator end-winding easier in spite of the forseen outside ring, i.e. to remove the inside ring and outer ring without having to disassemble the array of bars itself, the outside ring is located directly in the area of a bracket portion interconnecting the inner and outer bars, which bracket portion defines at the end area of the cone substantially a cylindrical surface such that, after the unscrewing of the tie-rods, the outside ring located at the end area can be pulled off. The outside ring and the inside ring as well are, however, additionally locked axially such that a disassembling remains relatively cumbersome. In order to benefit by a simpler disassembling procedure after all the relatively weakly dimensioned outside ring is arranged in such a manner at the cylindrical end area portion of the arrangement of bars that the radial bracing with the inside ring proceeds only at the end area of the bars, the axially much longer cone-shaped portion of the array of the bars is not braced further up to the body of the stator or core, respectively. The result thereof is a only limited rigidity of the stator end-winding system in spite of the massive inside ring.